1. Field of Art
The present invention relates to a secondary surveillance radar (SSR) making use of an extended squitter received from aircraft.
2. Description of Relevant Art
FIG. 1 illustrates an SSR 1a installed at a ground station. It has a transmitter-receiver 12 adapted for transmission of signals as interrogations to transponders 20 boarded on aircraft 2, and for reception of signals as replies transmitted from transponders 20 on aircraft 20 in response to interrogations. The transmitter-receiver 12 implements various processes under control of a signal processor 13a. The SSR 1a is adapted for such signal transmission and reception to acquire necessary information for air traffic control.
There are two types of transponders: an ATCRBS (Air Traffic Control Radar Beacon System) transponder that has been used since ever, and a mode S transponder that has been developed later than the ATCRBS transponder. They employ different signals for transmission and reception. In FIG. 1, the SSR 1a is an SSR mode S (secondary surveillance radar mode S) system cooperative with both ATCRBS transponder and mode S transponder, so it is adaptive as an SSR for surveillance, even for ATCRBS aircraft (aircraft with an ATCRBS transponder on board), as well as for mode S aircraft (aircraft with a mode S transponder on board).
As illustrated in FIG. 2, the SSR mode S 1a has the transmitter-receiver 12 including a transmission-reception selector 121 for selection between signal transmission and reception, a transmitter 122 for transmission of interrogations, and a receiver 123 for reception of replies. Further, the SSR mode S 1a has the signal processor 13a including a transmission controller 131 for controlling the transmission of interrogations, a mode S reply processor 136 for processing replies received from mode S aircraft, an ATCRBS reply processor 133 for processing replies received from ATCRBS aircraft, a surveillance processor 137 making use of received replies for preparation of target reports to survey flights of aircraft, and a channel controller 134 for controlling transactions in all-call time periods and roll-call time periods.
More specifically, as illustrated in FIG. 3, the receiver 123 includes a Σvideo detector 123a for detecting Σvideo from received signals, a digitizer 123b for digitizing detected Σ video, a Δvideo detector 123c for detecting Δvideo, a digitizer 123d for digitizing detected Δvideo, a Ωvideo detector 123e for detecting Ωvideo, and a digitizer 123f for digitizing detected Ωvideo.
Further, as illustrated in FIG. 3, the mode S reply processor 136 includes a mode S reply detector 136a for detecting, from among digitized signals from the digitizers 123b, 123d, and 123f, mode S replies transmitted from mode S aircraft, and a mode S reply analyzer 136b for analyzing detected mode S replies. On the other hand, the ATCRBS reply processor 133 includes an ATCRBS reply detector 133a for detecting, from among digitized signals from the digitizers 123b, 123d, and 123f, ATCRBS replies transmitted from ATCRBS aircraft, and an ATCRBS reply analyzer 133b for analyzing detected ATCRBS replies.
Further, the surveillance processor 137 includes a target report preparer 137a adapted to make use of results of analyses by the analyzers 133b and 136b, for preparing target reports.
The SSR mode S 1a is configured, as illustrated in FIG. 4, for time division into all-call time periods Ta and roll-call time periods Tr according to, among others, prescribed repetition periods and coverage area, to proceed with processes for acquiring ATCRBS aircraft and mode S aircraft. More specifically, the SSR mode S 1a has the transmission controller 131 adapted in all-call time periods Ta for transmission of all-call interrogations to acquire ATCRBS aircraft and mode S aircraft, and in roll-call time periods Tr for transmission of roll-call interrogations to keep acquisition of mode S aircraft having been acquired in all-call time periods Ta, for a processing to enable acquisition of all aircraft within a coverage (refer to Japanese Patent Application Laid-Open Publication No. 2007-248296, and “Secondary Surveillance Radar, Michael C. Stevens, 1998, ISBN 0-89006-292-7”).
However, the SSR mode S 1a may have an increased number of aircraft within the coverage, which might overlap addition of a datalink function, which would constitute a difficulty in acquisition of all aircraft within the coverage, thus shedding light on the importance of efficient use of the periods Ta and Tr.
In the meanwhile, aircraft is configured to transmit, besides the reply to be transmitted in response to an interrogation transmitted from the SSR mode S 1a, an extended squitter for use in the ADS-B (automatic dependent surveillance-broadcast). The extended squitter is a signal that is formatted, as illustrated in FIG. 5A and FIG. 5B, with fields of contents including “CA (transponder capability)”, “AA (address announced: transponder mode S address)”, “ME (message extend squitter: aircraft position, velocity, etc)”, and “PI (parity/interrogator identifies)”. The SSR mode S 1a may thus receive extended squitters, but is configured to use no extended squitters it has received.
As having been described, there is a conventional SSR mode S 1a that may have an increased number of aircraft within a coverage, which might overlap addition of a datalink function, which would constitute a difficulty to acquire all aircraft within the coverage. On the other hand, there is an extended squitter to be received irrespective of replies, which is not made use of at the SSR mode S 1a. 
With this point in view, it is an object of the present invention to provide a secondary surveillance radar adapted for utilization of an extended squitter transmitted from mode S aircraft to perform surveillance of aircraft.